Integrated optical receiver architecture for high speed optical i/o applications

ABSTRACT

An integrated optical receiver architecture may be used to couple light between a multi-mode fiber (MMF) and silicon chip which includes integration of a silicon de-multiplexer and a high-speed Ge photo-detector. The proposed architecture may be used for both parallel and wavelength division multiplexing (WDM) based optical links with a data rate of 25 Gb/s and beyond.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to optical receiversand, more particularly, to integrated optical receivers with embeddedtapers for improved fiber alignment tolerances.

BACKGROUND INFORMATION

Efficient light coupling between an optical fiber and a siliconwaveguide is highly desired for silicon based photonic device andcircuit applications. Due to the high refractive index contrast ofsilicon waveguide systems, obtaining good fiber-silicon waveguidecoupling may be challenging.

In optical communication, information is transmitted by way of anoptical carrier whose frequency typically is in the visible ornear-infrared region of the electromagnetic spectrum. A carrier withsuch a high frequency is sometimes referred to as an optical signal, anoptical carrier, or a lightwave signal. A typical optical communicationnetwork includes several optical fibers, each of which may includeseveral channels. A channel is a specified frequency band of anelectromagnetic signal, and is sometimes referred to as a wavelength.

Technological advances today include optical communication at theintegrated circuit (or chip) level. This is because integrated circuitshave size advantages that are attractive in computer systems. Sometimesdesigners couple an optical signal (light) between two chips, between achip and a die in the system, or between two dies. This is traditionallyaccomplished using an optical fiber to couple light between waveguideson dies or chips.

One limitation of using the optical fiber to couple light betweenwaveguides on dies or chips is that this method of coupling tends to beinefficient. One reason is because of the physical size differencebetween the optical fiber and a typical waveguide on a chip or die. Theoptical fiber tends to much larger than the waveguide. Because of thesize difference the optical signal coupling efficiency is poor. That is,the light from the larger diameter optical fiber does not fit well intothe small waveguide. The result can be that received light levels are solow that individual bits in the data stream in the optical signal becomeindistinguishable. When this happens, the receiving component may not beable to recover the information from the data stream.

Coupling efficiency may be improved by attaching lenses to the opticalfiber or by placing a lens between the optical fiber and the waveguideto focus the optical signal into the waveguide. However, couplingefficiency is only fair using lenses. Other coupling methods result inefficiencies that are also fair at best.

This limitation also comes with another challenge such as efficientcoupling from the optical mode supported by the larger optical fiber tothe smaller optical mode supported by the waveguide. The mode is theoptical cross-sectional distribution of energy (Gaussian distribution)and is defined by the size of your waveguide (optical fiber, planarwaveguide) and the wavelength of the light. There is a large opticalmode in the larger optical fiber and a smaller optical mode in thesmaller waveguide.

Also coupling from an optical fiber to small on-die waveguides requiresvery precise alignment. This is typically accomplished with specializedprecise manual alignment procedures. Such specialized alignmentprocedures typically are very expensive and limit practical volumes.

Today, there is a fundamental problem existing for a low-cost multi-modefiber (MMF) based optical receiver for high speed applications. Toachieve high speed, for example 25 Gb/s and beyond, operation for aphoto-detector (PD), the active area of the detector is usually requiredto be small. However, to efficiently couple light from MMF into asemiconductor waveguide based chip that contains photo-detectors andpotentially an optical de-multiplexer, a large waveguide size is usedfor big misalignment tolerance needed for low cost passive alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and a better understanding of the present invention maybecome apparent from the following detailed description of arrangementsand example embodiments and the claims when read in connection with theaccompanying drawings, all forming a part of the disclosure of thisinvention. While the foregoing and following written and illustrateddisclosure focuses on disclosing arrangements and example embodiments ofthe invention, it should be clearly understood that the same is by wayof illustration and example only and the invention is not limitedthereto.

FIG. 1 is a cut-away side view of an integrated optical receiveraccording to one embodiment of the invention;

FIG. 2 is a view of the silicon on insulator (SOI) substrate for formingthe optical receiver shown in FIG. 1;

FIG. 3 is a side view of the SOI wafer illustrating etching of thetaper;

FIG. 4 is a side view of SOI wafer illustrating etching of the V-groovefor the mirror;

FIG. 5 is a side view of the SOI wafer illustrating deposition of anodile layer for the total internal reflection mirror;

FIG. 6 is a side view of the SOI wafer having a silicon wafer bonded ontop;

FIG. 7 is a side view of the SOI shown in FIG. 6 flipped over forfurther silicon photonic processing of the photo-detector (PD) andoptional grating as shown in FIG. 1;

FIG. 8 is a graph illustrating modeling optical loss for the V-groovemirror structure of the integrated optical receiver under single modeconditions; and

FIG. 9 is a graph illustrating modeling optical loss for the V-groovemirror structure of the integrated optical receiver under multi-modeconditions.

DETAILED DESCRIPTION

Described is an integrated receiver optical receiver architecture toaddress light coupling between a multi-mode fiber (MMF) and silicon chipas well as integration of silicon de-multiplexer and a high-speedphoto-detector. The proposed architecture can be used for both paralleland wavelength division multiplexing (WDM) based optical links with adata rate of 25 Gb/s and beyond.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Referring now to FIG. 1, there is shown the optical receiver accordingto one embodiment of the invention. The optical receiver 100 comprises asilicon wafer portion 102 on which a total internal reflection (TIR)mirror structure 104 is situated. A waveguide portion 106 comprises awide end into which light from a fiber, such as a multi-mode opticalfiber 108 may be input. The light may be focused through a lens 110. Thewaveguide 106 comprises a taper 112 where the waveguide narrows from thebottom. The TIR structure comprises a wedge 114 having a reflectivesurface to direct the light, traveling parallel to the substrate 102, isreflected upward to a high speed photo-detector 116 as indicated by thearrows. A silicon de-multiplexer 118 may also be optionally fabricatedinto the waveguide 106 as shown. For example the de-multiplexer maycomprise a diffraction grating, such as the etched Echelle gratingillustrated. The etched Echelle grating may be capable ofde-multiplexing both single-mode and multi-mode beams.

This integrated silicon chip shown in FIG. 1, may be fabricated on asilicon-on-insulator (SOI) substrate. For parallel link applications,the de-multiplexer may not be included. The silicon taper 112 input hasa height of 20-30 um at the wide end for efficient coupling between theMMF 108 and the chip with a plastic lens 110. The final waveguide 106height after the taper is ˜10 um. The taper may be fabricated, forexample by using a grayscale technology as described in Optics Expressvol. 11, no. 26, 3555-3561 (2003), herein incorporated by reference.

Note that the final waveguide size should probably not be small becauseof the possible modal filtering effect (optical loss) for a multi modebeam launched from the MMF 108. The TIR reflection mirror portion 104 isused to couple light from the waveguide vertically to a high-speedgermanium (Ge) detector 116 grown on top of silicon. The fabricationtechnique of such a Ge PD 116 is well established. Because the lightfrom the tapered waveguide incident into the Ge PD can be reflected fromthe metal contact on top of the Ge layer in the detector, double opticalpath is achieved in the Ge active region. This leads to higher quantumefficiency with a thinner Ge film for higher speed. The estimated speedof the detector with a Ge thickness of ˜1.5 um is >20 GHz, good for 25Gb/s applications.

FIGS. 2-7 illustrate the fabrication steps of the proposed integratedsilicon receiver chip according to one embodiment. Referring now to FIG.2, a silicon on insulator (SOI) wafer comprises a silicon substrate aburied oxide (BOX) layer 202 and a silicon handle layer 204 on the BOXlayer 202. In one embodiment, the Si layer 204 may be approximately20-30 um thick. This thickness of course may differ for differingapplications. A sacrificial oxide hard mask (HM) layer 206 may be on topof the Si layer 204.

In FIG. 3, the taper portion of the waveguide from FIG. 1 is etchedthrough the HM layer 206 and partway through the Si layer 204 to an etchdepth of approximately 10 um. The etched portion may be generallyrectangular at one end and tapered at the other. In FIG. 4, a V-groove400 may be further etched from the Si layer 204 for later forming of thewedge mirror 114 shown in FIG. 1. The V-groove 400 may be etched, forexample, with a Potassium Hydroxide (KOH) wet etch technique. TheV-groove 400 etch may in some embodiments reach the buried oxide (BOX)202 or leaves a thin silicon layer (0.5-1 um) for Ge growth later.

In FIG. 5, the etched trenches are filled with oxide 500 followed bychemical mechanical planarization (CMP). The oxide comprises the totalinternal reflection (TIR) mirror shown in FIG. 1.

In FIG. 6 the planarized wafer will be wafer-bonded with a separatesilicon wafer 102. After the wafer bonding, the original handle wafer200 of the SOI wafer will be removed. As shown in FIG. 7, the entireapparatus may be flipped over. Removal of the handle wafer 200 creates anew wafer with BOX 202 as a hard mask. The hard mask (HM) BOX layer 202may be used for further process of Echelle grating 118 and Gephoto-detector (PD) as shown in FIG. 1, the processing techniques ofwhich are well known in the art.

Referring now to FIGS. 8 and 9, the optical loss of the V groove mirrorstructure is modeled under single mode and multi mode launchingconditions, respectively. With a V-groove angle of 54.7°, the singlemode case of FIG. 8 shows that there is almost no optical loss. As shownin FIG. 9, for the multi-mode case with 0-5 modes launching there is lowat only −0.36 dB. It is also noted that even with a silicon layer of 1um un-etched for the V groove mirror, optical loss is still small.Namely, most of the light is reflected at the mirror facet and directedto the PD 116.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

1. An apparatus, comprising: a silicon substrate; a total internalreflection (TIR) mirror structure over the silicon substrate, the TIRmirror structure comprising a first portion and a second portion thickerthan the first portion; a taper portion transitioning between the firstportion and the second portion of the TIR mirror structure; a V-groovewedge integral to the second portion of the TIR mirror structure; awaveguide on top of the TIR mirror structure; and a photo-detector (PD)fabricated over the waveguide proximate to the V-groove wedge.
 2. Theapparatus as recited in claim 1 further comprising: a de-multiplexerformed in the waveguide in front of the V-groove wedge.
 3. The apparatusas recited in claim 1 wherein the photo-detector comprises a high speedGermanium photo-detector.
 4. The apparatus as recited in claim 2 whereinthe de-multiplexer comprises a diffraction grating.
 5. The apparatus asrecited in claim 4 wherein the diffraction grating comprises an etchedEchelle grating capable of de-multiplexing both single-mode andmulti-mode beams.
 6. The apparatus as recited in claim 1 wherein thewaveguide comprises an input end approximately 20-30 um in thickness. 7.The apparatus as recited in claim 6 wherein the thickness after thetaper is approximately 10 um in thickness.
 8. A method for fabricatingan integrated optical receiver comprising: providing a silicon oninsulator (SOI) wafer comprising a silicon handle layer, a buried oxide(BOX) layer, a silicon waveguide layer and a hard mask (HM) oxide layer;etching a taper in the HM layer and the silicon waveguide layer; etchinga V-groove in a portion of the silicon layer; filling taper and theV-groove with oxide to form a total internal reflection (TIR) mirrorstructure; planarizing the TIR mirror structure; bonding a silicon waferto the TIR mirror structure; flipping the SOI wafer over; removing thesilicon handle layer; and fabricating a high-speed photo-detector (PD)over the V-groove.
 9. The method as recited in claim 8, furthercomprising: fabricating a de-multiplexer in the silicon waveguide layer.10. The method as recited in claim 8 wherein the photo-detectorcomprises a germanium (Ge) photodetector.
 11. The method as recited inclaim 9 wherein the de-multiplexer comprises a diffraction grating. 12.The method as recited in claim 11 wherein the diffraction gratingcomprises an etched Echelle grating capable of de-multiplexing bothsingle-mode and multi-mode beams.
 13. The method as recited in claim 8wherein the taper comprises a wide end approximately 20-30 um inthickness and a narrower end approximately 10 um in thickness.
 14. Themethod as recited in claim 8 wherein the V-groove is etched to the BOXlayer.
 15. The method as recited in claim 8 wherein the V-groove isetched short of the BOX layer.
 16. An integrated optical receiversystem, comprising: a silicon substrate; a total internal reflection(TIR) mirror structure over the silicon substrate; a silicon waveguideover the TIR mirror structure, the silicon waveguide having an wideinput end that tapers to a narrower end, the input end to receive lightfrom a multi-mode fiber; a photodetector fabricated over a portion ofthe narrower end of the waveguide; and a wedge portion on the TIR mirrorstructure beneath the photodetector to reflect light up to thephotodetector.
 17. The system as recited in claim 16 further comprising:a lens between the multi-mode fiber and the silicon waveguide input end.18. The system as recited in claim 16, further comprising; ademultiplexer formed in the waveguide prior to the wedge.
 19. The systemas recited in claim 16 wherein the photodetector is a germaniumphotodetector.
 20. The system as recited in claim 18 wherein thedemultiplexer is an Echelle grating.